It is common practice to add chemicals to sewage in order to disinfect same prior to discharge thereof into the surrounding watershed. Recently, considerable attention has been directed to the on-site generation of such disinfecting materials, especially sodium hypochlorite, in electrolysis cells. Such on-site production is desirable owing to the problems and expense involved in shipping and storing dilute hypochlorite solutions and the danger in handling the more concentrated chlorine gas in urban areas. Such processes are particularly attractive in coastal locations where the salt values of seawater provide an economical source of the hypochlorite precursor, sodium chloride. Inland, electrolytes may be prepared from solid sodium chloride, whether pure or impure.
The electrolysis of seawater, brackish water, impure saltwater or other solutions containing hardness impurities leads to a pronounced problem, however, in the nature of the formation of heavy deposits on the electrode surfaces. It has been recognized, that calcium and magnesium impurities in the impure electrolytes can cause hardness deposits to build up on the cathode which leads to pronounced increases in operating cell voltages, thus decreasing cell efficiency and ultimately these buildups of hardness deposits grow to such a thickness as to impede or prevent electrolyte flow through the cell.
Various techniques have been proposed for removing the unwanted deposits from the cathode surfaces and these include mechanical removal of the deposits, acid washing of the cell to dissolve the deposits, intermittent air blasts to dislodge such deposits, intermittently halting electrolysis and flushing the cell to remove such deposits and many other techniques which have not been wholly satisfactory.
Typical prior art procedures are described in the patent literature. For example, U.S. Pat. No. 3,893,902 describes an anode-cathode arrangement for use in forming aqueous hypochlorite solution by seawater electrolysis with reduced formation of interfering deposits on the electrodes.
U.S. Pat. No. 3,974,051 describes a method of minimizing hardness-caused deposits on the cathodic surfaces of hypochlorite-producing electrolysis cells by utilizing extremely smooth cathodic surfaces in conjunction with high velocity electrolyte.
Full current reversal through the cell has also been utilized to remove electrode deposits.
None of the produced solutions have been totally satisfactory. Mechanical cleaning is expensive and time-consuming and acid cleaning can cause corrosion problems in the cell.
The procedures and arrangements described in the two abovementioned patents do not fully prevent hardness deposits and thus in time such deposits must be removed in order to regain cell efficiency. Current reversal, while useful in systems where anodes were changed frequently, is essentially ruled out in modern systems utilizing dimensionally stable anodes since such current reversal would quickly damage the anode. Additionally, many of these prior art proposals require stopping the operation of the cell which means decreased production and more expensive product.